J Cardiol 2007 May; 49（5）: 241 – 250

Comparison of Cardiovascular
Autonomic Responses in Elderly
and Young Males During Head-Out
Water Immersion
Masahiro ITOH, PhD
Yoshiyuki FUKUOKA, PhD＊1
Sunao KOJIMA, MD＊2
Haruo ARAKI, MD＊3
Norio HOTTA, PhD＊1
Tomohiro SAKAMOTO, MD＊2
Katsuhide NISHI, MD＊4
Hisao OGAWA, MD, FJCC＊2
Abstract ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Objectives. To investigate the cardiovascular autonomic responses to head-out water immersion in thermoneutral water. The effects of immersion levels（neck, chest, navel）
and breathing frequencies（4, 6, 10,
15 times/min）were compared with in ambient air and spontaneous breathing as a control.
Methods. Spectral analysis of heart rate variability was recorded in 11 young（mean age 20 ± 1 years）
and 11 elderly（mean age 68 ± 6 years）healthy male subjects during water immersion. Modeling was
employed to estimate the time course of low-frequency（LF）and high-frequency（HF）
power and the ratioof LF to HF power of heart rate variability.
Results. In the young group, stroke volume and cardiac output during navel level water immersion manifested a much greater increase than during chest and neck level water immersion. Systolic blood pressure
and total peripheral resistance decreased significantly in response to water immersion. The elderly group,
however, showed lesser attenuation of stroke volume and cardiac output. Ectopic arrhythmias occurred inonly the elderly. The elderly group showed significantly lower and higher amplitudes of HF and LF/HF
components of heart rate variability due to water immersion and breathing frequency changes.
Conclusions. These findings suggest differential changes in cardiovascular autonomic responses
between the young and elderly groups. These changes in integrative cardiovascular autonomic responses
may account for the increased risk of ectopic arrhythmias in elderly people during water immersion. Water
immersion model could be utilized to know circulatory regulation during bathing.
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────── J Cardiol 2007 May ; 49
（5）
: 241−250

Key Words■ Heart rate
■ Autonomic nervous system（power spectra analysis）（cardiovascular■ Cardiac output■ Elderly（water immersion）（vagal tone）
response）

──────────────────────────────────────────────
熊本大学医学部保健学科 生理機能検査学 : 〒 862−0976 熊本県熊本市九品寺 4−24−1 ; ＊ 1 熊本県立大学 環境共生学部，熊本 ; ＊ 2 熊本大学大学院医学薬学研究部 循環器病態学，熊本 ; ＊ 3 あらき循環器内科，熊本 ; ＊ 4 桜十字病院 内科，熊本
Department of Physiology, Kumamoto University School of Health Sciences, Kumamoto ; ＊ 1Laboratory of Applied and Environmental
Physiology, Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto ; ＊ 2Department of
Cardiovascular Medicine, Kumamoto University Graduate School of Medical Sciences, Kumamoto ; ＊ 3Araki Heart Clinic, Kumamoto ;＊4
Division of Internal Medicine, Sakurajyuji Hospital, Kumamoto
Address for correspondence: ITOH M, PhD, Department of Physiology, Kumamoto University School of Health Sciences, Kuhonji 4−24−1, Kumamoto, Kumamoto 862−0976 ; E-mail : ito@gpo.kumamoto-u.ac.jp
Manuscript received July 25, 2006 ; revised January 10 and March 2, 2007 ; accepted March 3, 2007

241

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Itoh, Fukuoka, Kojima et al

INTRODUCTION
Upright head-out water immersion（WI）shifts
venous blood to the central vascular compartment
and heart and subsequently increases cardiac preload from the legs and abdomen.1−5）In young nor・ and stroke volume
mal subjects, cardiac output（Q）（SV）increase during WI, and by increasing thedepth of immersion, the cardiac filling pressures
also increase.6）However, there is little information
about arterial blood pressure adjustment inresponse to WI. Significant decreases occur in systolic（ SBP）, diastolic（ DBP）and mean arterial
blood pressures（ MBP）
, 7）whereas no change in
MBP and an increase in arterial pulse pressure
occur during WI.8）Thus, the effects of WI on bloodpressure remain unclear.
In elderly people, cardiovascular autonomic dysfunction is likely to contribute to hemodynamic
impairment in orthostatic testing and WI, with anattenuated heart rate（HR）response and augmented
vascular responses.9,10）The dynamic capacity ofcardiac autonomic regulation decreases and vascular sympathetic regulation becomes augmented
with increasing age.11）The factors influencing thesignificant elevation of blood pressure during phase
Ⅰ rehabilitation are age, physical deconditioning,
imbalance of autonomic nervous activity and anxiety. 12）Therefore, a graded increase in WI could
lead to a graded increase in cardiac distention andpossible arrhythmias in elderly people.
The present study simultaneously measured
blood pressure, HR, ・Q and SV, and assessed autonomic cardiovascular control during a graded WI
protocol in young and elderly males using spectral
analyses of HR variability. Most previous studies
used spectral techniques based on the fast Fouriertransform. Spectral analysis of HR variability has
been used frequently as a noninvasive tool for theassessment of the autonomic cardiovascular control
in recent years.13−15）However, Fourier transform is
insufficient to estimate the precise power spectral
density from short time series data. Accordingly,
the present analysis was carried out using the
MemCalc method,16）which allows reliable analysisof the low-frequency（LF ; 0.04−0.15 Hz）component at a minimum interval of 30 sec. There is a
general consensus that the high-frequency（ HF ;0.15−0.4 Hz）component of HR variability, whichreflects what is commonly termed respiratory sinus
arrhythmia, represents a reasonably good index ofparasympathetic nerve activity. 13）On the other
hand, there remains considerable controversy as towhether the LF component of HR variability represents only sympathetic nerve activity or a combination of both sympathetic and parasympathetic modulation of HR, so that an alternative index is theratio between LF to HF representing an index of
sympathetic nerve activity.
Recently, HR variability analysis has been used
to evaluate autonomic nervous activity, and studies
have reported the relationship between coronary
spasm and autonomic activity. 17,18）The present
study compared the effects of changing breath frequencies during WI as well as graded WI levels
between elderly and young individuals.
SUBJECTS AND METHODS
Subjects
Eleven young（mean age 20 ± 1 years）and 11
elderly（mean age 68 ± 6 years）healthy males participated in the clinical study（Table 1）. The young
subjects were well-trained and competitive swimmers in a university swimming team. Some elderly
subjects had been treated for peptic ulceration, gout
and gallstones. Three of the elderly subjects had
hypertension and were treated with thiazide diuretics. No subject had a history of cardiovascular diseases and all were healthy as indicated by medical
history, physical examination, blood pressure（＜
140/90 mmHg）and 12-lead electrocardiography（ECG）. The experimental protocol was approvedby the ethics committee of Institutional Review
Board of the School of Medicine, Kumamoto
University. All subjects provided written consent
for their participation after they were fully
informed about the study. The investigation was
performed in compliance with the Declaration of
Helsinki.
Study protocol
Subjects underwent four experimental sessions inbalanced random order wearing a swimsuit : 1）
upright position non-immersed in the tank（air）, 2）
upright position at WI to the level of the sternoclavicular notch（neck）, 3）upright position at WI tothe level of the xiphoid process（chest）, and 4）
upright position at WI to the level of the spina iliaca anterior inferior（navel）. The tank was pre-filledwith thermoneutral warm tap water. The water temperature was maintained at 34 °C throughout the
study. At the end of WI, the subjects were immediJ Cardiol 2007 May; 49（5）: 241 – 250

Heart Rate Variability During Water Immersion

Age
(yr)

Height(cm)

Weight Vital capacity
(kg)
(ml)

Elderly
A

72.0

154.1

44.8

2,440

B

59.0

174.0

71.0

4,430

C

67.0

161.2

59.8

2,350

D

57.0

170.6

77.2

3,680

E

70.0

164.4

68.3

3,080

F

76.0

165.2

57.5

2,430

G

74.0

160.0

67.6

2,420

H

64.0

169.5

62.5

3,430

I

72.0

171.3

71.4

2,380

J

73.0

163.0

58.1

3,030

K

65.0

163.5

72.6

3,540

Mean±SD

68.1±6.2 165.2±5.8 64.6±9.2 3,019±691

Young
M

19.0

170.5

57.2

4,210

N

19.0

163.0

65.5

4,260

O

21.0

164.5

80.0

4,710

P

23.0

176.0

74.0

4,600

Q

21.0

165.0

54.0

4,500

R

18.0

175.0

69.0

3,870

S

19.0

167.2

64.0

4,400

T

20.0

172.5

62.0

4,770

U

19.0

175.0

63.8

3,980

V

19.0

175.0

65.0

4,440

W

19.0

171.0

59.0

4,380

Mean±SD19.7±1.4 170.4±4.8

65±7.4 4,375±281

ately dried and covered with towels to prevent coolC. No sysing. All tests were performed at 24−27 °tematic difference in the room and water temperatures between the trials was observed.
Each test consisted of five different respiratory
periods of 3 min each, with spontaneous breathing
and four voluntary changes in the respiratory period
from 4 times/min（4 C）to 6 times/min（6 C）, then
10 times/min（10 C）and 15 times/min（15 C）, while
maintaining constant tidal volume（ TV ;1,500 ml）
. The rhythm of breathing was controlled
under each set of conditions by the subjects listening to a tape recording of instructions regarding theduration of inspiration and expiration. The subjects
themselves also controlled TV by watching a flow
meter and following instructions from theresearchers. The study protocol is shown in Fig. 1.

J Cardiol 2007 May; 49（5）: 241 – 250

bf（times/min）

Table 1 Physical characteristics of participating subjects

243

20
15
10
5
0

SP

6C

4C
3

6

15C

10C
9

12

15

Time（min）

Fig. 1 Study protocol
Each test consisted of five different respiratory
periods of 3 min each, with spontaneous breathing（SP）and four voluntary changes in the respiratory period from 4 times/min（ 4 C）to 6
times/min（6 C）, to 10 times/min（10 C）and 15
times/min（15 C）. Subjects underwent four
experimental sessions in balanced random order,
air, neck, chest and navel.
bf ＝ breathing frequency.

Measurements and data analysis
Ventilatory flow was measured with a hot wire
flow meter（RM-300, Minato Medical Sciences）.
The fractional concentrations of O 2 and CO 2 insamples drawn from the face mask were analyzed
with a zirconium solid electrolyte oxygen analyzer
and an infrared carbon dioxide analyzer, respectively（Vmax29c, Sensor Medics）. This system wascalibrated with a 2-l syringe, fresh air and an accu. The time delay
rately analyzed gas（O2 15%, CO2）
for gas flow to the analyzers was compensated for.
The HR was measured continuously by transistortransistor logic signal intervals synchronized with
the R-wave of ECG by CM5 leads.
At each level of immersion, ECG, ・Q, SV, SBP,
DBP, and total peripheral resistance were recorded
continuously, whereas oxygen saturation（SaO2）
was measured every minute. Blood pressure wasrecorded with automatic oscillometric equipment（Jentow CS, Nihon Colin）at the heart level on thearm resting alongside the body. Special precautions
were taken to ensure that the position of the cuff
relative to the heart did not changed during thestudy. ・Q was determined during the last minute ofeach stage using the equilibrium acetylene
rebreathing technique and pulse counter method.
All data were adopted during the last minute ofeach 3-min stage. In all subjects, the ECG data
were recorded using one channel, lead CM5 on a
DAT tape recorder during the study and analyzed
with a computer-based system（AG901, NihonKoden）. The R-R interval was measured by detect-244

Itoh, Fukuoka, Kojima et al

ing the peak of each R-wave using a built-in analog-to-digital converter with an interval resolutionof 8 msec. The data for each subject were transferred to a personal computer and stored on a 3.5inch MO disk. These data were analyzed by the
MemCalc system（Suwa Trust）. We obtained the RR interval（msec）as the time domain index of HR
variability. The MemCalc system is a linearizedversion of the nonlinear least squares method forfitting analysis in the time domain, combined withthe maximum entropy method for spectral analysis
in the frequency domain. 19）The HR variability
power spectrum was divided into HF（0.15−
0.40Hz）and LF（0.04−0.15Hz）components. The
HF component has been used to infer parasympathetic nervous activity,20,21）whereas the LF/HF ratioof HR variability was defined as a better indicatorof sympathetic nerve activity, because the LF component is typically related to a combination ofparasympathetic and sympathetic influences.22−24）
For the analysis of 1/f fluctuations of HR, the power
spectral band was focused on the range from 0.04
to 0.15. The slope of the 1/f fluctuations of HR was
calculated by the expression on a log 10（frequency）− log10（power density）scale ; i.e., theslope of the regression line between the log10 of frequency between 0.04 and 0.15Hz and the log10 ofpower density of the R-R spectrum was calculated.
The LF, HF and LF/HF values are reported as natural logarithms（ln）.
Statistical analysis
All data are expressed as mean ± SD. Statisticalanalysis was performed by two-way analysis ofvariance（ANOVA）. When F values were significant, individual comparisons were made using the
Bonferroni test. The paired Student s t-test wasused to evaluate differences between two trials. A
probability（p）value of ＜ 0.05 was regarded as statistically significant. The Fisher exact probability
test and unpaired t-test were used to compare categorical and continuous variables between the young
and elderly groups, respectively. The natural logarithm transformation（ln）of each variable, which
produces near-normal distribution, was applied
before statistical analysis was performed, because
the distribution of the frequency domain measurements of HR variability was extremely skewed.

RESULTS
Cardiovascular responses
Fig. 2 shows the circulatory responses to WI. HR
tended to increase gradually with increased breathing frequency in both groups, but the increase was
not significant. There was significant bradycardia
during WI in the young group compared with air（navel : 61.0 ± 9.7 beats/min, chest : 59.3 ± 7.0
beats/min, neck : 61.5 ± 10.7 beats/min, air :84.0 ± 7.7 beats/min ; spontaneous breathing, p ＜0.01）, but no such change was observed in theelderly group. No significant differences were
found between different WI levels, navel, chest andneck, in the young group. WI also produced a significant increase of SV at the navel level in theyoung group compared with air, even under spontaneous breathing（ navel : 126.1 ± 39.7 ml,
air : 79.5 ± 15.5 ml ; p ＜ 0.01）, resulting insignificantly increased ・Q at the navel level compared with chest and neck levels in the young group
under 6C（navel : 10.0 ± 4.1 l, chest : 6.1 ± 1.2 l,neck : 6.9 ± 1.5 l ; p ＜ 0.01）. The elderly group
showed no evidence of any significant increase in
SV and ・
Q at any WI levels or with changing
breathing frequency.
SBP, DBP and total peripheral resistance weresignificantly higher in the elderly group comparedwith the young group at any WI level and breathing
frequency（ p ＜ 0.01）. In the young group, both
SBP and DBP increased gradually with increased
breathing frequency but were not significantly different between different WI levels. In the elderlygroup, SBP and DBP were not significantly different between different breathing frequency and WIlevels（Fig. 3）.
No arrhythmias were observed during WI in theyoung group. In contrast, single supraventricular
extrasystoles, including premature atrial and junctional complexes, were the most prevalent arrhythmias and were observed in 7 of the 11 elderly participants during WI. These arrhythmias occurred
throughout the entire study protocol, in air and WI,
spontaneous and enforced by breathing frequency,
where some of the arrhythmias seemed to be related to respiratory arrhythmias.
Spectral analysis
Changes in LF, HF and the LF/HF ratio werenoted in both the young and elderly groups. Fig. 4
shows the changes in LF, HF and LF/HF ratio in
J Cardiol 2007 May; 49（5）: 241 – 250

Heart Rate Variability During Water Immersion

ElderlyHR（beats/min）

120

100

80

80

60

60

40

SP

4C

6C

10C

15C

100

SV
（ml）

40

SP

4C

6C

10C

15C

150

60
100
40
50

20
0

Q
（l/min）

Air
Navel
Chest
Neck

200

80

・

Young

120

100

245

SP

4C

6C

10C

15C

0

8

16

6

12

4

8

2

4

0

SP

4C

6C

10C

15C

0

SP

4C

6C

10C

15C

SP

4C

6C

10C

15C

Fig. 2 Changes in heart rate
（upper）, stroke volume（middle）
and cardiac output（below）
with difference of breathing frequencies（spontaneous, 4, 6, 10, 15 times/min; SP, 4C, 6C, 10C, 15C）
andhead-out water immersion levels（air, navel, chest and neck）
Left and right figures show the elderly and young males, respectively. Data are expressed asmean ± SD. ＊ p ＜ 0.05, ＊＊ p ＜ 0.01 vs each breathing frequency in air within group.
HR ＝ heart rate ; SV ＝ stroke volume ; ・Q ＝ cardiac output. Other abbreviations as in Fig. 1.

the two groups. Compared with the young group,
the elderly group had significantly lower LF（p ＜0.05）and HF（p ＜ 0.01）powers of HR variability
under all conditions, including different WI levels
and breathing frequency. In contrast, the LF/HF
ratio of HR variability in the elderly group was significantly higher at breathing frequency of 4C and
6C than in the young group（p ＜ 0.05）.
DISCUSSION
The results of this study demonstrated that graded WI to navel, chest and neck levels induced adaptive changes in cardiovascular responses in the
J Cardiol 2007 May; 49（5）: 241 – 250

young group, and that there was less adaptation to
the conditions in the elderly group. ・Q and SV weremarkedly lower in volume in the elderly groupcompared with the young group. Interestingly, total
peripheral resistance was significantly higher
throughout the study in the elderly group than in
the young group. WI was associated with arrhythmias in the elderly subjects. The typical cardiovascular autonomic response to WI occurred by aging.
In the young group of this study, HR markedly
decreased during all WI（diving bradycardia）. Both・
Q and SV, however, significantly increased only
between air and navel level, suggesting that

246

Itoh, Fukuoka, Kojima et al

SBP（mmHg）

180

180

160

160

140

140

120

120

100

100

80

80

DBP
（mmHg）

60

SP

4C

6C

10C

15C

SP
120

100

100

80

80

60

60

40

40

20

TPR
（mmHg/l/min）

60

120

20
SP

4C

6C

10C

15C

80

40

60

30

40

20

20

10

0

SP

4C

6C

Air
Navel
Chest
Neck

Young

Elderly

10C

15C

0

4C

6C

10C

15C

SP

4C

6C

10C

15C

SP

4C

6C

10C

15C

Fig. 3 Changes in systolic blood pressure（upper）, diastolic blood pressure（middle）
and total peripheral resistance（below）
with differences of breathing frequencies（spontaneous, 4, 6, 10, 15
times/min; SP, 4C, 6C, 10C, 15C）
and head-out water immersion levels（air, navel, chest andneck）
Left and right figures show the elderly and young males, respectively. Data are expressed asmean ± SD. ＊ p ＜ 0.05, ＊＊ p ＜ 0.01 vs each breathing frequency in air within group.
SBP ＝ systolic blood pressure ; DBP ＝ diastolic blood pressure ; TPR ＝ total peripheral resistance. Other abbreviations as in Fig.1.

myocardial contractility significantly increased
only during WI to the navel level, resulting in higher SV. An increase in cardiac filling pressure withgraded WI induces increases in both central venous
pressure and arterial pulse pressure.25）On the other
hand, we found no differences in cardiovascularresponses depended upon WI levels. One of thereasons for this discrepancy is likely to be the lower
increase in central venous pressure response to
WI.25） Firstly, in our study protocol, a stable andgiven TV（1,500 ml）against hydrostatic pressure
may stimulate more stretch-reflexes in the lung andthorax and subsequently might exert a suppressing
influence on the cardioinhibitory center（Bainbridge
reflex）. 26）Under conditions of WI to the navel
level, however, venous return could easily andrapidly increase from the lower body into the thorax because of the absence of transmitted hydrostatic pressure. Alternatively, the significant differencein SV between air and navel WI under SP might
J Cardiol 2007 May; 49（5）: 241 – 250

Heart Rate Variability During Water Immersion

Air
Elderly

In HF（msec2）

12

6

＊＊
＊＊
＊＊ ＊ ＊＊ ＊＊＊＊ ＊ ＊＊＊ ＊ ＊

4C

6C

＊＊
＊＊ ＊
＊

10C

In LF
（msec2）

4C

6C

15C

12
9

9
＊
＊

＊
＊ ＊
＊＊

＊ ＊ ＊＊＊ ＊ ＊＊＊＊＊＊＊ ＊ ＊
＊

10C

15C

6
3

3
SP

4C

3.0
In LF/HF

SP

15C

12

2.5

＊

＊
＊

6C

SP

4C

6C

10C

15C

SP

4C

6C

10C

15C

3.0
＊
＊

＊
＊＊
＊＊

2.5
2.0

2.0
＊ ＊

＊

＊

1.5
1.0

1.0
0.5

10C

6
3

SP

1.5

Neck

9
＊
＊＊
＊

3

6

Chest

Young

12

9

Navel

247

SP

4C

6C

10C

15C

0.5

Fig. 4 Mean changes in each power of R-R interval variability（msec2）and ratio of LF/HF with difference of breathing frequencies（spontaneous, 4, 6, 10, 15 times/min; SP, 4C, 6C, 10C, 15C）
and of head-out water immersion levels（air, navel, chest and neck）
between the elderly（left）and young（right）
groupsUpper, middle and below figures show ln HF, ln LF, ln LF/HF, respectively. Data are expressed asmean ± SD. ＊ p ＜ 0.05, ＊＊ p ＜ 0.01 vs each breathing frequency or water immersion levels
between the young and elderly groups.
LF ＝ low-frequency ; HF ＝ high-frequency. Other abbreviations as in Fig.1.

indicate that only SV or together with the hydrostatic pressure to the thorax might have greater contributions to increased central venous pressure.
Another reason might be the acquired characteristicof the young subjects through physical training.
The fitness and/or training of the subjects can modify the renal and hormonal responses to WI. 27）
Trained young subjects probably have an intricate
volume regulatory system of cardiovascularresponses.28）Under conditions of central hypervolemia induced by chest and neck levels of WI,
central blood volume might be maintained by arterial baroreceptors and autonomic responses in subserving volume and circulatory homeostasis.
J Cardiol 2007 May; 49（5）: 241 – 250

In contrast, HR, SBP, DBP, SV and ・
Q in theelderly group changed less than in the young group,
indicating reduced ability to withdraw from vagal
influence 29）and/or decrease arterial baroreflex
responses30）with hydrostatic pressure in the elderly. Controversy still exists with regard to the
response of blood pressure to WI in humans.1,3,7）
Changes in cardiovascular autonomic regulation areassociated with aging under various conditions,
including WI.9,10）However, our results indicatedthat despite the added hydrostatic pressure, bothautonomic and neuroendocrine activities were hardly necessary in circulatory homeostasis in the elderly group because of the lesser increase in central

248

Itoh, Fukuoka, Kojima et al

venous pressure. Blood pressure in the elderlygroup during WI also appropriately responded tomaintain homeostasis, 31）probably because theelderly have permanent increase of blood pressure
with age-related lower vascular compliance.
Moreover, total peripheral resistance in the elderlygroup was increased more during WI, possibly due
to age-dependent reduction in suppression of muscle sympathetic nerve activity.32）
Spectral analysis of HR variability might allow
the detection of momentary changes in bothparasympathetic and sympathetic activity functions.
At rest, elderly subjects exhibit increased sympathetic nerve activity and decreased parasympathetic
nerve activity as well as impaired arterial baroreflex
function.11,30）We found a marked increase in HF
components of HR variability in the young groupby graded WI compared with the elderly group,suggesting that parasympathetic deactivation ofcardiac control during WI is important for adaption
in the young group. In contrast, the LF/HF ratio in
the elderly group during both 4C and 6C was significantly higher than that in the young group. In
the elderly group, 4C or 6C of breathing frequency
and 1,500 ml of TV could have elicited sympathetic
nerve activity by overwork of their pulmonary
capacity. HF power represents the vagal function of
HR caused by the respiratory cycle and the depth of
respiration.20,21）However, the LF/HF ratio of HR
variability as a complex parameter is not a sample
index of sympathetic activity.11）
In the present study, the rhythmic changes in
activity likely elicited rhythmic variations in HR
from lower to higher breathing frequency. R-R
interval lengthened much less with breathing frequency change than with level of WI. The vagal
control of HR in humans and respiratory sinus
arrhythmia caused by reduced vagal efferent activity during the inspiration phase of the respiratory
cycle is well documented.33）Therefore, our present
findings demonstrated that bradycardia produced byparasympathic nerve activity at lower breathing frequency was predominant during WI in the younggroup. On the other hand, there was less parasympathic nerve activity in the elderly group with either
changing breathing frequency or graded WI. It is
important to emphasize that our results were consistent with the age-dependent attenuation ofparasympathic nerve activity previously reported.11）
Diving is well known to elicit brady-arrhythmias
as well as premature atrial and ventricular contrac-

tion or even ventricular tachycardia. 34−36） The
potential dangers of the vagal response associatedwith diving include temporary sinus arrest.37）In thepresent study, the most prevalent arrhythmias wereobserved in 7 of 11 elderly subjects during WI.
However, these arrhythmias are not likely to besolely due to increased cardiac overload（pre-load）
or the Frank-Starling mechanism and baroreflex
pathway. Total peripheral resistance, the so-called
after-load, increased from air to all levels of WI,
indicating that peripheral factors may also be related to the occurrence of arrhythmias. Therefore, atleast in part, these ectopic arrhythmias during WI
may be induced by augmented sympathetic nerve
activity at lower breathing frequency, and by agedependent attenuation of parasympathic nerve
activity, as supported by the higher LF/HF ratio of
HR variability at 4C and 6C, and lower HF powerof HR variability throughout this experimental protocol. In addition, absence of augmentation of cardiac vagal activity in dogs is a risk factor for ventricular tachyarrhythmias.38）WI could be responsible for the increased risk of ventricular arrhythmias
in elderly people despite the thermoneutral watertemperature having little effect on cardiovascularresponses.
Study limitations
This study does not attempt to evaluate rigorously the cardiovascular autonomic responses by difference of individual lung volume. The subjects
were forced to maintain constant 1,500 ml of TV.
Therefore, the TV ranged from 31 to 68% for each
vital capacity of the subjects. Limited data in thisstudy could not allow any clear conclusion for thedifference of reflex response to lung inflation mediated thoracic stretch receptors. 39）Further study
should settle the different effects of individual
physiological response accompanied by breathing.
CONCLUSIONS
The main findings of the present study are as follows. Graded WI led to different changes in cardiovascular responses between elderly and young people, with less adaptation in the elderly group. The
typical response linked to parasympathetic deactivation was age-related. The occurrence of arrhythmias depends much more on autonomic activity
elicited within the periphery as the dominant factor
than central factors during WI in elderly people.

J Cardiol 2007 May; 49（5）: 241 – 250

Heart Rate Variability During Water Immersion

Acknowledgments

249

Miss K. Inoue and Miss K. Iwasaki. This study was partly supported

We gratefully acknowledge the expert technical assistance of

要by a grant from the Descent Sports Foundation（to Dr. Itoh, 2001）.

約

若年者および高齢者における浸水時の心血管系への自律神経応答の比較

目

伊藤

雅浩

福岡

義之

小 島

堀田

典生

坂本

知浩

西

淳

荒木

春夫

勝 英

小川

久雄

的 : 中性温の浸水が心血管系に及ぼす自律神経の応答を検討するため，異なる水深（頸部，胸部，臍部）および呼吸頻度（4，6，10，15 回/min）の影響を陸上および自由呼吸との比較から研究した．
方法 : 浸水時における 11 例の男性若年者（平均年齢 20 ± 1 歳）および 11 例の男性高齢者（平均

年齢 68 ± 6 歳）の心拍変動を周波数領域から解析した．その方法では，心拍数の低周波成分（LF），
高周波成分（HF），およびその成分比（LF/HF）を用いて評価した．
結果 : 若年者群において，浸水位が臍部での 1 回拍出量と心拍出量は胸部や頸部よりも顕著な増加を示した．また，最大血圧と末梢血管抵抗は浸水で有意に減少した．しかしながら，高齢者群
では 1 回拍出量および心拍出量が減少傾向を示した． 浸水中は高齢者群のみに期外収縮の出現あるは悪化が検出された．浸水と呼吸変化によって，高齢者群には有意な低 HF と高 LF/HF が心拍変動から認められた．
結論 : 本研究結果で，中性温の浸水が若年者群と高齢者群間における心血管系への自律神経応

答に異なる変化が示唆された．加齢による心血管系への自律神経応答の変化が，浸水中の高齢者に期外収縮の発現を増大させるのではないかと推察された．浸水の研究は，入浴時の循環調節の知見として活用できよう．

J Cardiol 2007 May; 49（5）: 241−250

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